Heparin and related low molecular weight heparin derivatives remain critical drugs for the prevention and treatment of thrombotic conditions. Concerns relating to the stability and sufficiency of the porcine intestinal mucosa supplies from which most heparin is currently derived have spurred an interest in identifying other sources for heparin. Commercial quantities of heparin may be obtained from cow and sheep intestine, and heparin or heparin-like material has been identified in a number of other species. Being biologic in nature, these heparins exhibit unique structural and functional profiles. This chapter summarizes our current understanding of these heparins.